Mental Leap

What apes can teach us about the human mind

At the opening of Stanley Kubrick’s 1968 film 2001: A Space Odyssey, a group of apes hovers around an object that has suddenly appeared in the desert. The sleek, black, rectangular object is five times as tall as the apes and clearly crafted by intelligent beings. The apes approach it with caution, and one animal runs a timid hand along the clean edges that glimmer in the sunlight.

A chimpanzee uses a stick as a fishing rod at Arnhem Zoo in the Netherlands. Apes’ use of tools has suggested to some researchers that certain human mental capabilities trace back 14 million years to common ancestors of apes and humans. Frans de Waal
GREAT GRAPE. An ape carries pipes that it can use to retrieve grapes locked in a container. In an experiment, apes showed the foresight to save such tools for future use. J. Kalbitz/MPI-EVA
METHOD ACTING. A chimpanzee pokes a device to release food. Even though most apes learned that the trigger could be either poked or lifted, they usually chose the method of their group leader—a sign of conforming to cultural norms. de Waal /Yerkes National Primate Research Center
SAY WHAT? A bonobo named Nyota points to a symbol on a device called a Lexigram. Some apes can communicate with people by pointing to Lexigram symbols, which represent objects or actions, such as chase.

Suddenly, something clicks in the ape’s mind. The sight of a sophisticated innovation has launched dormant aptitudes, and the ape has realized that a large bone can be used as a weapon to advance its kind. Standing more erect than before and brandishing the weapon, the ape attacks another group.

The film then fast-forwards through the remainder of cognitive evolution in a flash: The bone, tossed into the air, becomes a space station floating through the cosmos.

Too bad the camera didn’t cut instead to a laboratory at the Max Planck Institute in Leipzig, Germany, where scientists are discovering details about apes’ brains that could fill in some of the movie’s multimillion-year gap. Researchers at the institute, Josep Call and Nicholas Mulcahy, recently demonstrated that apes possess a surprising understanding of tools and even make plans to use them. In fact, the study suggests that planning skills go back 14 million years to ancestors of apes and people.

Two other knacks once considered unique to humans have recently turned up in apes. A team of scientists watching apes at the Yerkes Primate Center at Emory University in Atlanta reports that the animals conform to cultural norms—that is, they do a task the same way that others in a group do it, even if an alternative method exists.

Another team studying apes raised at the Language Research Center at Georgia State University, also in Atlanta, concludes that young animals play make-believe, an activity considered an early sign of linguistic abilities.

These studies represent an effort to understand some of the human mind’s fundamental mechanisms. While recognizing a difference in the degree of human and ape capabilities, “I don’t think there’s a difference in kind,” says Call.

Investigating these differences could reveal “the processes that led to our position,” says Carel van Schaik, director of the Anthropological Institute and Museum at the University of Zurich in Switzerland. “We can build a bridge across the [ape-human] gap without denying that there are some really big differences.”

Planning of the apes

For Call and Mulcahy, the most fitting metaphor for ape cognition isn’t A Space Odyssey but an Estonian folktale. In that story, a young girl dreams of attending a party where she can’t eat dessert because she has no spoon. The next night, the girl takes a spoon to bed, in case the dream recurs.

The scientists placed bonobos and orangutans in a similar scenario. Previous studies had clearly shown that apes use tools to satisfy an immediate need. In the wild, for example, they frequently transport rocks to smash open nuts. But the researchers wanted to know whether apes would hold on to a tool that they wouldn’t need until much later.

Before the test, the apes had learned how to retrieve a bunch of grapes locked inside a container. The apes had to insert a thin plastic pipe into a slot, as though the pipe were a key. In the test, Call and Mulcahy led the apes, one at a time, into a test room where each animal saw the grape-holding container and four objects: a dowel, a plastic dish, a bowl, and the slot-fitting pipe.

Here, the experiment went beyond observations of tool use. A Plexiglas panel blocked the container, so the ape couldn’t retrieve the grapes, even with the right tool. Each animal was free to take away an object when it left the room and wasn’t permitted to return for an hour.

By then, the researchers had removed the Plexiglas and all the tools. An ape that had selected the right tool, removed it from the test room, and brought it back after the delay could then help itself to grapes. In other words, the successful animals planned ahead.

The test was performed 16 times for each of three bonobos and three orangutans. On average, each ape solved the task 7 times, with one ape solving it 15 times and another only twice. The animals improved slightly as they repeated the test.

The best-performing bonobo and orangutan were brought back for a more difficult test. In 16 additional trials, each of these two apes visited the test room and then was kept out of it overnight, yet about half the time they carried the appropriate tool into the room the next morning.

But, the most impressive display of sophisticated thinking, Mulcahy says, came from an unexpected incident. An orangutan brought back the wrong tool and then shaped it into a usable key.

To learn whether the apes were carrying the tools simply by force of habit or because they enjoyed doing so, Call and Mulcahy performed another test, in which they repeatedly removed the grapes after a first showing behind the Plexiglas. That way, the apes could learn that there was no reason to haul the tool around—after all, why bring a spoon to a party without desserts?

Sure enough, two of the apes in this trial never brought along the pipe. The two other animals in this trial seldom did so, the researchers reported in the May 19 Science.

Mulcahy argues that the animals’ delayed success depends on them making a realization similar to that of the girl in the folktale, who had to fail—at least in her dream—before she could plan ahead.

Since bonobos and orangutans both solved the tasks, Call and Mulcahy concluded that the cognitive seeds for planning were already in place at least 14 million years ago, when a common ancestor to these apes and humans lived.

Van Schaik comments that he frequently sees evidence of planning from apes in the wild. For example, orangutans often give “long calls” to let others know the direction in which they’re going to travel a couple hours later, he says. While this action isn’t the same as considering retirement options, the long calls suggest that apes might at least plan their afternoons.

But some other researchers are skeptical of the new data. Clutching a tool to get grapes doesn’t demonstrate planning, says Daniel Povinelli, who is director of the Cognitive Evolution Group at the University of Louisiana at Lafayette.

The apes weren’t necessarily thinking about the task ahead when they selected the pipe. They had been trained to use it but had no experience with the other tools, which “might as well be books of Shakespeare,” Povinelli says.

Moreover, the apes might have learned to associate having the pipe with success in getting the grapes, which is a simpler mental function than planning. Povinelli points out that most of the apes didn’t consistently bring back the pipe until halfway through the experiment, which is what he’d expect for the association explanation. “The fact they carry [the tool] isn’t evidence to their conception of the future,” he says.

Despite Povinelli’s criticism, a commentary accompanying the paper says that the study might someday be considered a landmark. Call and Mulcahy demonstrate “the most extensive foresight yet in nonhuman primates,” writes cognitive psychologist Thomas Suddendorf of the University of the University of Queensland in Australia. “This provides a starting point from which we can begin to reconstruct the evolution of the human mind,” he says.

Swamp smarts

Other recent experiments also suggest humanlike cognition in apes. Last year, researchers working at Emory found evidence that animals conform to cultural influences in the same way that people do (SN: 9/3/05, p. 158: Available to subscribers at Chimps ape others to learn tool use). Such cultural conformity, for example, motivates a person to bow in greeting in Japan but to shake hands in the United States.

In the study, two groups of apes learned to operate a device that releases food when a trigger is either lifted or poked. In each group, subordinate apes predominantly got the food in the same way—a lift or a poke—that their group leader did. Because they occasionally pulled instead of poked, or vice versa, the animals had learned both techniques during repeated trials. But when the apes were retested 2 months later, they still overwhelmingly used their leader’s method, the team reported in the Sept. 29, 2005 Nature.

“To find that some aspects of culture, however rudimentary, can be found in other animals,” says study coauthor Frans de Waal, “means our ancestors—maybe even our animal ancestors—had cultural traditions. We know that these cultural traditions were essential to their survival.”

But the evolutionary importance of culture might go beyond survival, says van Schaik. He thinks that culture, as a mechanism for transmitting knowledge, played a critical role in hominid cognitive advancement. In fact, cultural interaction in early humans might have catalyzed the heightened intelligence that we possess today, he argues in the April Scientific American.

Van Schaik came to this conclusion after observing a group of wild orangutans in the hot, insect-ridden swamps of Sumatra. Although most orangutans don’t use tools in the wild, he says, he found a population of orangutans that fashioned sticks to penetrate the husk of a nourishing but prickly fruit. By contrast, other groups of orangutans that lived nearby risked injury by opening the prickly fruit with their hands or avoided it.

The stick-using orangutans also showed other innovative behaviors that the other groups lacked, and this population was the only one in which adults foraged together.

Van Schaik theorizes that the apes’ frequent interactions enabled less-skilled adults to learn an innovative behavior and teach it to their children. Over time, such cultural dissemination advances the overall intelligence of the population, he asserts.

“There seems to be a link between culture and intelligence, in that animals that are more culturally advanced, or have broader cultural repertoires, actually turn out to be smarter,” van Schaik says. “That’s not terribly surprising if you talk about people, but you don’t expect it to be true for nonpeople.”

Thus spake Panpanzee

As innovations grew in complexity, the primitive communication techniques used by early apes could have likewise grown. “People speculate why we have language,” de Waal says. “One answer could be, it facilitates the transmission of a lot of knowledge.”

A new study tackles the evolutionary roots of language by observing the playful behavior of young apes. In people, both language and pretending are considered outgrowths of a general symbolic function, says Patricia Greenfield of the University of California, Los Angeles. If apes develop pretending behavior by advancing through the same stages that children do, Greenfield and her colleagues say, then some shared ancestor probably possessed this general symbolic function.

Human children demonstrate five stages of pretending, the highest being the treatment of toys as animate objects, which shows up after age 2. To study whether apes follow the same path, the researchers analyzed playful interactions between five apes—three bonobos and two chimpanzees—and their caregivers. Most interactions took place when the apes were 2 to 5 years old.

The young apes indeed progressed through the stages of pretend play in the same way that human children do, the researchers report in the July-September Cognitive Development.

The most impressive example of pretending occurred when a female chimp named Panpanzee, then 4 years old, pretended to groom a doll, says study leader Heidi Lyn of the University of St. Andrews in Scotland. Panpanzee then pretended to pluck bugs off her body and feed them to the doll, offering some bugs to the caregiver, Liz, as well. Liz offered some imaginary bugs back to Panpanzee, who pretended to eat them.

Because people, bonobos, and chimpanzees had a common ancestor 5 million years ago, symbolic function—and thus the developmental roots of language—extends back at least this far, the authors conclude.

In the study, the young apes often began pretending on their own and were boosted to a higher level by the responses of their caregivers. The researchers call this process scaffolding. The beginnings of a cognitive skill, such as pretending or using language, might exist without being used for that purpose, but scaffolding puts the mental wheels in motion, Greenfield says.

In a particular environment, the seeds of cognition can mature, she proposes. So, while apes may not naturally use symbolic function for language, they do so in settings where they are trained to communicate through a device called a Lexigram.

Pretend play and Lexigram use may be evidence of a generalized symbolic function out of which language could have grown in the subsequent millions of years, Greenfield says.

Povinelli is more cautious. He worries that when researchers focus on traits shared by ape and human ancestors, they ignore differences between the species that could reveal as much or more about ape and human cognition.

“The danger is that we use our folk psychology to reconstruct the minds of apes according to our own models of human beings,” Povinelli says. “We never get at what it’s really like to be an ape, and we never fully appreciate what it means to be human.”

Without a vast study of many animal species, it’s difficult to suggest that any given quality in people emerged from an ancestor shared with apes, says Simon Reader of Utrecht University in the Netherlands, who studies innovative behavior in animals.

“If you only test chimpanzees and find a behavior in common with humans, you still don’t know if the behavior is found across primates or the animal kingdom in general,” says Reader.

Povinelli would like to see researchers demonstrate that apes consider abstract notions, such as the fundamental difference between colors and shapes. Such experiments have been tried, unsuccessfully, in monkeys, he notes. Discovering the true limits of ape intelligence will show more about humanity, says Povinelli.

“We know that apes can learn to use tools properly,” he says. “We’ve known that for 100 years.” The real question is whether ape behavior is the result of abstract considerations—perhaps about the future, the past, or love—or simply a reaction to a situation.

“Yes, they’re thinking,” says Povinelli. “But what are they thinking about?”

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